The present invention relates to a method for reading radiographic image information which is accumulated and stored in a radiographic image conversion panel having a photostimulable phosphor, and specifically to an efficient reading apparatus for radiographic image information.
A radiographic image such as an X-ray image is widely used for medical purposes. As a method to obtain the radiographic image, a radiographic method is used in which a fluorescent substance layer (fluorescent screen) is irradiated with radioactive rays passed through a subject, visible light is generated thereby, and a silver salt photosensitive material is irradiated with the visible light and developed like an ordinary photographing method. However, recently, a method has been invented in which the radiographic image information is obtained without using a radiographic film made of a silver salt photosensitive material. In this method, radioactive rays which have passed through a subject are absorbed into some kind of a fluorescent substance, then the fluorescent substance is excited by, for example, light or heat energy, thereby radiographic ray energy which is accumulated in the fluorescent substance by the foregoing absorption is irradiated as a fluorescent light, and the fluorescent light is detected, so that the image can be formed. Specifically, this method is disclosed in, for example, U.S. Pat. No. 3,859,527, or Japanese Patent Specification Open to Public Inspection No. 12144/1980. In these specifications, a photostimulable phosphor is used, and a radiographic image conversion method in which visible light or infrared rays are used as excitation light, is disclosed as follows. A radiographic image conversion panel is used in which a photostimulable phosphor layer is formed on a support; the photostimulable phosphor layer is irradiated with the radioactive rays which have passed through a subject, and radiographic ray energy corresponding to radiographic ray transmittance of each portion of the subject is accumulated to form a latent image; then, the photostimulable phosphor layer is scanned by the foregoing stimulation excitation light, and thereby the radiographic ray energy which is accumulated in each portion of the radiographic image conversion panel is radiated and converted into light; and a light signal according to the intensity of the light is detected by a photoelectric conversion element such as a photo-multiplier, a photodiode, or the like so that radiographic image information can be obtained.
The radiographic image information thus obtained is not further processed, or image-processed when spatial frequency processing or gradation processing is conducted in a real time, and is outputted onto a silver salt film or a CRT to be visualized, or stored in an image memory such as a semiconductor memory, a magnetic memory, an optical disc memory or the like, and after that, it is read out as necessary from these image memories and outputted onto a silver salt film or a CRT to be visualized.
According to the foregoing radiographic image conversion method, a radiographic image having a lot of informations can be obtained by a smaller radiographic ray exposure amount than that by a conventional radiographic method. Accordingly, the radiographic image conversion method is extremely useful for direct radiographing such as X-ray radiographing which is used particularly for medical diagnosis.
However, the foregoing method has a problem in which radiographic ray energy, which is stored in a radiographic image information recording medium by radiographic ray radiation, is largely decreased as time goes by. (This phenomenon is called "fading".) Therefore, as shown in FIG. 1, the intensity of the image signal is lowered as time passes from a time (t.sub.0) at which the image has been irradiated by a radioactive ray, to a time (t.sub.1 l) at which the radiographic image is read out by excitation light, or to a time (t.sub.2) at which the radiographic image has been read out. Here, (I.sub.0 I.sub.1)/I.sub.0 will be referred to as a fading value, where I.sub.0 is the signal intensity of the image signal at a time t.sub.0 at which the image has been irradiated, and I.sub.1 is the signal intensity at time t.sub.1.
That is, as time elapses from the time (t.sub.0) to the time (t.sub.1) from which radiographic image reading is started, the intensity of the image signal is lowered, and thereby a ratio of signal (S)/noise (N) is lowered, so that image quality of the radiographic image obtained as above is deteriorated. Further, as time elapses from the time (t.sub.0) to the time (t.sub.2) at which the image reading has been completed, the intensity of the obtained image signal is lowered, and thereby a difference between the magnitude of the image signal obtained at the start of reading and that of the image signal obtained at the completion of reading is caused, so that a uniform image can not be obtained.
To cope with this problem, a reading apparatus in which CsI:Na is used as a fluorescent substance layer and is maintained under the temperature of 180K is disclosed in (1) Japanese Patent Publication Open to Public Inspection No. 123829/1986.
Further, a reading apparatus in which the fluorescent substance layer is maintained under the temperature of 250K to 300K is disclosed in (2) Japanese Patent Publication Open to Public Inspection No. 180941/1988.
However, in the method (1) described above, a large cooling apparatus is necessary since the cooling temperature is extremely low, and thereby the reading apparatus becomes large and complicated.
Further, in the method (2), although it is described that fading is small in the practical low temperature region, the photostimulable phosphor which is used in this reading apparatus is only generally described in the publication, and only RbBr:Tl is disclosed as a specific example of the fluorescent substance with a superior property.
In view of the foregoing, the object of the present invention is to provide a radiographic image reading apparatus in which fading of a signal read out from an energy image accumulated in a radiographic image recording medium, is satisfactorily small in a practicable low temperature region, and a radiographic image of good quality can be obtained.